1. Field of the Invention
The present invention relates to an air pressure detection device for tire which takes into account the temperature at a predetermined portion inside a tire. More specifically, to an air temperature in an air chamber inside the tire or the temperature of a tread portion, a carcass portion or a shoulder portion or the like of the tire.
2. Description of Background Art
A tire air pressure alarm device for measuring the air pressure of a tire and informing the driver of the air pressure even during operation of the vehicle is known, for example, as set forth in Japanese Patent Laid-open No. Hei 10-44726, entitled xe2x80x9cTire Air Pressure Alarm Device.xe2x80x9d As set forth in FIG. 1, Japanese Patent Laid-open No. Hei 10-44726, a tire air pressure alarm device is disclosed in which a valve stem 10 (symbols used in the publication are used here) is fitted to a wheel rim 1. A case 2 is disposed at a lower portion of the valve stem 10. A transmission unit 7 includes a pressure detecting portion 3, a signal processing circuit 4 and an electric cell 5 contained in the case 2. In this device, the value of the pressure directly detected by the pressure detecting portion 3 is subjected to signal processing and the processed signal is transmitted to a reception unit.
The temperature of a tire and the temperature of air in an air chamber inside the tire are raised due to friction with the road surface or the like according to the operating conditions. Attendant on the temperature rise, the air pressure in the air chamber inside the tire is also raised. Therefore, since the air pressure influenced by temperature is detected in the above-mentioned invention, it has been difficult to judge whether the tire air pressure during running is maintained at a recommended air pressure.
Accordingly, it is an object of the present invention to provide an air pressure detection device for tire which takes temperature variation into account.
In order to attain the above object, an air pressure detection device for a tire includes an air pressure sensor for measuring the air pressure of a tire, a temperature sensor for measuring the temperature at a predetermined portion inside the tire, a reduced air pressure arithmetic means for calculating a reduced air pressure from the air pressure measured by the air pressure sensor and the temperature measured by the temperature sensor, and an alarm signal generating means for generating an alarm signal when the reduced air pressure calculated by the reduced air pressure arithmetic means is lowered to or below a predetermined first threshold value.
The temperature at a predetermined portion inside the tire means the temperature of air in the air chamber inside the tire or the temperature of the tire itself at a tread portion, a carcass portion, a shoulder portion or the like or a similar in-tire temperature. In judging whether the air pressure of a tire is maintained at a recommended air pressure, it is effective to take into account the temperature at a predetermined portion inside the tire.
From this point of view, the air pressure of the tire is measured, and, at the same time, the temperature at a predetermined portion inside the tire is measured, then a reduced air pressure is calculated, and an alarm signal is generated based on the reduced air pressure. Therefore, it is possible to know the air pressure affected little by temperature, and to generate an alarm with high accuracy.
The present invention is directed to a reduced air pressure Pm calculated by the reduced air pressure arithmetic means is calculated by the following formula:
Pm=Pxxe2x88x92K(Txxe2x88x92T0) 
where Px is the air pressure of the tire measured by the air pressure sensor, Tx is the temperature at the predetermined portion inside the tire measured by the temperature sensor, T0 is a recommended temperature corresponding to a recommended air pressure for the tire, and K is a predetermined constant.
The reduction or conversion of air pressure taking temperature into account can be performed according to Boyle-Charles"" formula which states that (absolute pressure)/(absolute temperature) is constant. However, as will be described in detail, it has been found that this conversion produces a large error.
In view of this, the reduced air pressure is determined by the formula: Pxxe2x88x92K(Txxe2x88x92T0). As a result, the error becomes minute, and it is made possible to monitor the air pressure more accurately.
In the present invention, the alarm signal generating means generates an alarm signal when the decrease per unit time of the air pressure of the tire measured by the air pressure sensor is decreased below a predetermined second threshold value.
Since the alarm signal is generated when the decrease per unit time of the air pressure of the tire measured by the air pressure sensor is decreased below the predetermined second threshold value, it is possible to swiftly detect leakage of air from the tire and to generate an alarm.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.